Radiation Damage & Risks

State what DNA stands for and describe its basic structure

Deoxyribonucleic acid contains 2 strands of nucleotides held together by hydrogen bonds, forming a double helix structure.

Break down the structure of DNA

DNA contains a backbone of alternating deoxyribose sugars and phosphate as well as nitrogenous bases attached to the deoxyribose sugar. 

Describe a nucleotide

A deoxyribose sugar, a phosphate and a nitrogenous base.

Name the complementary nitrogenous bases.

• Adenine and thymine

• Cytosine and guanine

Describe why DNA is significant in the effects of radiation

DNA is the principal target of radiation damage

Explain the difference between direct and indirect effects of radiation

Direct effects involves radiation such as alpha and beta particles that directly interact with and damage DNA. Whereas indirect effects involves and intermittent stage in which radiation such as x and gamma rays that interact with other molecules to damage DNA. 

Explain the process of indirect action

X and gamma rays interact with atoms producing secondary electrons via Compton or photoelectric processes. Secondary electrons then interact with water molecules, forming a highly reactive hydroxide molecule that interacts with and damages DNA. 

Explain the 2 types of free radicals

• Spurs - 95% of radicals with 100eV energy containing ~3 ion pairs

• Blobs - 5% of radicals with up to 500eV energy containing ~12 ion pairs

Explain the difference between single-strand breaks and double-strand breaks.

Single-strand breaks involves a break to 1 DNA strand which can be rapidly repaired and therefore has minimal biological effects. Whereas double-strands breaks involves a break to both DNA strands and is mostly irreversible, therefore can lead to cell death, mutations and cancer.

Explain the types of lethal and non-lethal DNA damage

Non-lethal

• Terminal Deletion - permanent loss of broken fragment

• Symmetric Translocation - 2 breaks occur in 2 seperate chromosomes in which broken fragments are exchanged

Lethal

• Dicentric Formation - 2 centromeres occur in which broken ends join incorrectly

• Ring Formation - broken ends join together to form a ring

• Anaphase Bridge - a break occurs in 2 seperate chromatids which join together and cannot be separated during anaphase, therefore chromatids are stretched between poles.

Describe the difference between Non-Homologous End Joining and Homologous Recombination Repair in double-strand break repair.

• Non-Homologous End Joining occurs within G1 phase of the cell cycle without a sister chromatid

• Homologous Recombination Repair occurs within the S and G2 phases of the cell cycle with a sister chromatid. 

Explain the phases of the cell cycle

• M - mitosis, where cell division occurs

• Interphase

  • G1 - cell growth and organelle replication takes place

  • S - synthesis, where DNA synthesis occurs

  • G2 - further growth and final preparations before cell division

Explain what occurs in mitosis

Made up of 5 phases including prophase, metaphase, anaphase, telophase and cytokinesis in which the 2 copies of DNA separate, splitting the nucleus and forming 2 identical daughter cells.

Explain what occurs in G1 phase 

After mitosis, daughter cells receive energy and enzymes in order to grow and prepare for S phase. 

Explain what occurs in S phase

DNA synthesis occurs in which DNA from each daughter cell is replicated to form 2 exact copies within each cell.

Explain what occurs in G2 phase

After S phase, cells continue to grow and prepare for mitosis

Explain what G0 phase is

Phase outside of the cell cycle in which cells permanently or temporarily exit the cell cycle and are considered dormant as they are either in insufficient/unfavourable conditions or are required to perform their basic functions.

Explain the purpose of checkpoints within the cell cycle

Checkpoint genes which monitor cell cycle progression to ensure no cellular or DNA damage, preventing mutations during mitosis. 

Explain telomeres

Structures containing repetition of TTAGGG bases that cover and protect the terminal ends of chromosomes.

Explain the purpose telomeres

During cell division, telomeric DNA is lost from the lagging strand and telomeres shorten. After a certain amount of telomeric DNA is lost, cell death is induced. 

Define apoptosis

Physiological cell suicide that occurs within stressed, hypoxic, redundant and aged cells.

Describe tumours in terms of mitosis and apoptosis

Tumours occur when mitosis exceeds apoptosis

Explain the effects of radiation to DNA and other cell components.

To DNA:

• Lack of cell control function

• Increased proliferation

• Abnormal cell cycle progression

• Cell death at the next mitosis

To other cell components:

• Loss of cell functions

• Toxic waste production

• Cell death

State the annual effective whole body dose limits for radiation workers over the age of 18 and between 16-18, the public and patient carers. 

• Radiation Worker over 18 - 20mSv per year averaged over 5 years

• Radiation Worker 16-18 - 6mSv per year 

• Public - 1mSv per year 

• Patient Carer - 5mSv per year 

State whether the annual effective dose limits are in addition to background radiation or not.

They are in addition to background radiation which is ~3mSv per year.

Explain the dose limits for patients

Patients have no dose limits, however the dose they receive for a medical radiation procedure must follow ALARA principles.

State and explain the main sources of background radiation

• Cosmic Rays - high energy particles moving through space that produce secondary electrons that reach earth and are inhaled

• Building Materials - contain radioactive elements that decay into radioactive radon gas

• Earth’s Crust - decays into radioactive radon gas 

• Internal - elements part of the body as well as foods and liquids ingested

Explain the risks of radiation with cancer

An annual effective dose of 1Sv will give an additional radiation induced cancer risk of 1 in 20,000.

Explain what is classified as non-ionising radiation

Anything less than visible light on the EM spectrum which is <12eV.

Name and state the range of the 3 types of UV light

• UV A - 3.1 - 3.94eV

• UV B - 3.94 - 4.43eV

• UV C - 4.43 - 12.4eV

Explain the effects of the different types of UV radiation

• UV A - benign, causes sun burn can result in premature aging (skin wrinkling and decreased elasticity 

• UV B - can be benign and can cause melanoma, is required for vitamin D metabolism

• UV C - can cause melanoma, is germicidal 

Explain mitotic death

When an irradiated cell dies before the next mitosis

Explain how the diameter of DNA effect how radiation can damage it

Radiation that has a shorter frequency and wavelength than 2nm can destroy DNA as it is smaller than the DNA itself, therefore radiation with a greater wavelength is non-ionising.

Name and explain the 3 types of radiation exposures

• Planned - for a medical radiation procedure

• Accidental - not planned, eg. Radiation spills or being exposed to radioactive patients

• Occupational - working in medical radiations

Explain when it is appropriate for a pregnant women to undergo a medical radiation procedure

When the risk of not completing the procedure is greater than the risk of radiation exposure

State the dose limit of a pregnant medical radiation worker

1mSv, same as the public

Explain the changes in a medical radiation worker’s role once announcing her pregnancy

• No hot lab work

• Must carry 2 OSL’s, one for the baby and one for herself

• Baby’s OSL is measured monthly

Explain the when radiation risks are the most significant during pregnancy and why

During the first 10 days of pregnancy as the embryo only contains a few unspecialised cells, which if damaged, can be resorted into the maternal tissue and lost.

Explain the radiation risks during organogenesis (weeks 3-8)

Organs begin to develop and any damage to DNA in these cells can result in organ malformation/dysfunction.

Explain the radiation risks to a growing foetus’ CNS

• Sensitive weeks 8-25

• >100mGy can result in IQ reduction

• >1Gy can result in severe mental retardation

Explain the radiations risks of cancer to a growing foetus

>10mGy increases cancer risk by 40%

Explain when the termination of pregnancy considered based on radiation risk

>100mGy can cause foetal damage in which the mother can make the decision to terminate the pregnancy, however circumstances differ based on stages of pregnancy.

Explain how radiation risks change while moving through the pregnancy

Risks surrounding organ malformations and dysfunctions decrease as the pregnancy progresses as organs may have already formed.